There is a tendency to build check valve in a reinforced manner, in order to avoid for instance the penetration of water in the fresh water supply duct, in case of underpressure.
The mounting of check valves can become critical when they are meant to be used in connection for instance with single-lever mixing faucets. When, in these fittings, the cold water side is closed off and only the hot water side is open, the cold water check valve being also closed at the same time, the water between the latter and the closed cold water valve seat of the mixing faucet can be heated so intensely, that system pressures result which can damage the fitting. This danger is particularly great at pressures over 16 bar.
From U.S. Pat. No. 4,561,559 a check valve for fuel delivery is known, in whose valve closing member a pressure-relief valve is provided, whereby the valve closing member of the check valve as well as the pressure-relief valve are subjected to spring action. Such a safety device is of course indispensable for a device meant to distribute fuel and for its maintenance. High costs are acceptable in view of the risks.
However when it comes to use in sanitary water fittings the safety device has to be manufactured at low cost, with regard to the material as well as to production time, and practically maintenance free.